The invention concerns a method for joining workpieces, in particular press fittings to a pipe, in which a pressing device is placed with its at least two pressing jaws in the open position against the joining point of the workpieces, and the pressing jaws are then moved with respect to one another by means of a jaw drive, while the workpieces deform plastically, into a terminal pressed position in which, at the latest, the application of power to the press jaws ends. The invention further refers to a pressing device for joining such workpieces, having at least two pressing jaws and a jaw drive for moving the pressing jaws from an open position into a terminal pressed position, and having a control device for controlling the jaw drive, the pressing device being configured so that the application of power to the press jaws ends, at the latest, when the terminal pressed position is attained.
It is known, in order to join pipes, to use sleeve-like press fittings which, in order to produce a pipe joint, are slid over the pipe ends and then pressed radially together, both the press fitting and the pipe being plastically deformed. Pipe joints of this kind and the pertinent press fittings are known, for example, from DE-C-11 87 870, EP-B-0 361 630, and EP-A-0 582 543.
Pressing takes place with the aid of pressing devices such as are known in various embodiments, for example from DE-C-21 36 782, DE-A-34 23 283, EP-A-0 451 806, EP-B-0 361 630, and DE-U-296 04 276.5. The pressing devices have a pressing jaw unit having at least two or sometimes more pressing jaws, which during the pressing operation are moved radially inward to form a substantially closed pressing space. A jaw drive is provided for this movement, for example in the form of an electric motor or a hydraulic pressure cylinder or a combination of the two.
In the case of the known pressing devices, the jaw drive always travels toward a specific, constant terminal force. Terminal force limiters, for example in the form of an overpressure valve in the case of a hydraulic pressure cylinder, a torque coupling in the case of a rotating drive, or an overcurrent release in the case of an electric motor, are provided for this purpose. To ensure that compete pressing takes place in all circumstances, the terminal force is set relatively high. The reason is that inaccuracies in the terminal force limiter have a strong effect on the terminal force that can actually be attained, since terminal force limiters do not measure directly the force proceeding from the jaw drive, but rather a converted magnitude which represents only a fraction of the actual drive force. The high terminal force leads to wear on the bearing points of the pressing jaws, and all the parts acted upon by the jaw drive must be dimensioned accordingly.
The problems described above even if the jaw drive is matched to the particular pressing jaw unit connected to it, and also to the workpieces to be pressed therewith. Usually, however, a specific jaw drive is used for a jaw set made up of a plurality of pressing jaw units which are configured for pressing different diameters. For this purpose, the jaw drive can easily be detached from the particular pressing jaw unit and attached to another pressing jaw unit. In order for the jaw drive to be usable for all the pressing jaw units of a jaw set, the jaw drive and the terminal force limiter are designed so that the jaw drive and the terminal force achievable therewith are sufficient for pressing with even the largest pressing jaw unit.
The problems described above occur even with these pressing units. They become more serious as the pressing jaw units become smaller, and thus as the deformation work to be performed decreases. The terminal force at which the jaw drive is shut down is then far greater than the actual force needed. As a result, the pressing jaw units for small workpiece diameters must be grossly overdimensioned, i.e. they are heavier and more costly than necessary, and are subject to severe wear. But since it would be even more expensive to provide a matched jaw drive for each pressing jaw unit (not to mention transport problems), this is necessarily accepted.
In the case of the pressing devices described above, the jaw drive acts on a ram with spreader rollers, which during the pressing operation is moved out toward the pressing jaw unit. In the process, the spreader rollers travel between two pivot levers, arranged in mirror-image fashion, and spread them apart by acting on opposing spreading surfaces on the pivot levers. The pressing jaws, which are moved together by the spreading of the pivot levers, sit at the free ends of the pivot levers.
The pressing device described in DE-A-43 21 249 operates in the reverse fashion, i.e. during the pressing operation, the ram is pulled toward the jaw drive. At the beginning of the pressing operation, the spreader rollers are located between the pivot levers close to their pivot axes. The spreading surfaces enclose a gap which tapers in a V-shape toward the jaw drive and which, during the pressing operation, is expanded by the spreader rollers, thus reducing the angle of the V. When the terminal pressed position is reached, the spreader rollers enter a section between the pivot levers in which the spreading surfaces run parallel to one another, so that no further force is transferred onto the pressing jaws. A limit switch which automatically shuts down the motor is located in this section.
It is the object of the invention to make available a method which puts less load on the moving parts of the pressing device, and causes less wear during pressing. A further object is to provide a suitable pressing device therefor.
With regard to the method, the object is achieved, according to the invention, by the fact that the power of the jaw drive is controlled in such a way that, at least toward the completion of pressing, the pressing jaws have less kinetic energy than without a power controller.
This proposal is based on the recognition, already part of the invention, that when the full, i.e. uncontrolled, power of the jaw drive is present, considerable kinetic energy builds up in the parts being moved by it, in particular in the pressing jaw unit, principally in the first phase when a nonworking stroke is being performed. The less the work of deformation to be performed, the less such energy builds up.
The result of the reduction in kinetic energy according to the invention is that the maximum force which acts on the parts moved by the jaw drive is substantially reduced, and ideally is the same as the maximum force to be applied when deforming the workpieces.
Preferably the jaw drive is shut down, at the latest, when the terminal pressed position is reached. As a result of position-dependent shutdown of the jaw drive, in conjunction with the reduction in kinetic energy (which ideally equals zero at the completion of pressing), no high forces, resulting from kinetic energy still present at that time, occur after shutdown of the jaw drive. The press jaw units can accordingly, in particular in the lower size range, be of much lighter dimensions, and wear is also considerably less.
As far as the type of power profile is concerned, the basic inventive idea can be carried out in various ways. It is conceivable, for example, for the pressing jaws to be acted upon at full power from the outset, but for the jaw drive to be shut down before the terminal pressed position is reached. The work of deformation can then be performed by the kinetic energy stored in the moving parts, such that said energy partially or entirely dissipates up until the terminal pressed position. Shutdown of the jaw drive can then be effected as a function of the size of the pressing unit, in such a way that the less work of deformation to be performedxe2x80x94i.e. as a general rule, the smaller the diameter of the workpieces being pressed, and the softer their configurationxe2x80x94the earlier it occurs.
As an alternative thereto, provision is made for the pressing jaws, in a first phase of the pressing operation in which a nonworking stroke is being performed, to be acted upon by a power profile in which the power is reduced in such a way that the pressing jaws have, at least at the end of this phase, less kinetic energy than without power limiting. Preferably, the power profile in this context should be such that the kinetic energy of the pressing jaws in the first phase does not exceed that at the completion of pressing. This proposal is based on the observation that, especially in the nonworking stroke phase, a high kinetic energy builds up which is then not, or only insufficiently, dissipated in the actual pressing phase. If the kinetic energy in the first phase is kept low by reducing the power of the jaw drive, it then no longer leads, or leads only insignificantly, to an increase in kinetic energy in the pressing phase. In this context, it may additionally be appropriate to configure the power profile in such a way that the kinetic energy of the pressing jaws falls off toward the completion of pressing, by again reducing the power output of the jaw drive in the terminal phase.
Even with the power profile described above, it may be advantageous for the application of power to the pressing jaws to be discontinued even before the terminal pressed position is reached, so that the remainder of the work of deformation is performed by the kinetic energy which is then still stored in the moving parts. Ideally, power application should be discontinued in such a way that the kinetic energy equals zero when the terminal pressed position is reached.
It is understood that particularly optimum conditions are achieved when the power profile is adapted steplessly to the particular work to be performed, in particular with the goal, according to the invention, of having as little kinetic energy as possible at completion of pressing. Great progress is, however, achieved even if the power profile changes in steps, even if only one step is present, for example in the form of a first phase during which operation occurs at reduced power, and a second phase in which the pressing jaws are acted upon by full power. The switchover from the first to the second phase can then be adapted to the particular pressing jaw unit, advantageously in such a way that the less work of deformation to be performedxe2x80x94i.e., usually, the smaller the diameter of the workpieces and the softer their materialxe2x80x94the further the switchover is shifted toward the completion of pressing.
As far as the pressing device is concerned, there is provided in order to carry out the method described above a control device having a power control device which generates a power profile for the jaw drive such that the pressing jaws have, at least at the completion of pressing, less kinetic energy than without a power controller. The aforementioned alternatives of the method according to the invention can be implemented depending on the embodiment of the power control device. If the power profile is configured in two steps, the power control device can be embodied in such a way that the first phase extends into the deformation range, and to a greater extent, the less the work of deformation to be performed.
In terms of the physical configuration of the power control device, the general existing art in the field of power control of electric motors can be utilized. An electronic power controller is particularly suitable.
In a particularly preferred embodiment, a limit switch is provided in order to shut down the jaw drive, at the latest, when the terminal pressed position is reached. The limit switch can be triggered by the control device in such a way that the jaw drive is shut down before the terminal pressed position is reached, so as to utilize the kinetic energy still stored in the moving parts for residual deformation of the workpiece, with the goal of making the greatest possible use of the kinetic energy when the terminal pressed position is reached.
In a particularly preferred embodiment, the limit switch has a sensor for sensing the position of the pressing jaws, which is coupled to the control device in such a way that the power control device is triggered as a function of the position of the pressing jaws. According to this feature, therefore, the position of the pressing jaws is sensed directly and utilized for the power control device. It is particularly advantageous in this context if the sensor is coupled to a signal generator for the power control device, specifically and in particular if the sensor and optionally the signal generator are arranged on the pressing jaws themselves. This makes it possible to adjust the sensor and signal generator to the particular pressing jaw unit in each case so that a power profile which is optimally adapted to that pressing jaw unit is achieved, as a function of the actual position of the pressing jaws. The sensor (and optionally the signal generator) should be arranged on two adjacent pressing jaws in such a way that it senses the position of said pressing jaws with respect to one another, preferably in the region of the opposing end surfaces.
This can be done, for example, by the fact that the sensor has in each case a movably mounted feeler which coacts with a stop against which the feeler arrives as the pressing jaws close. The feeler can be installed on one pressing jaw, and the stop on the adjacent pressing jaw. It is also advantageous if the stop is configured adjustably in the movement direction of the feeler.